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An experiment is identified as a very carefully conducted and controlled study.
Confounding occurs when variation from one source is mixed (or confused) with variation from another source so that it is impossible to know whether effects are due to the impact of either variable separately or some combination of them.
Control consists of methods researchers use to remove or hold constant the effects of nuisance variables.
Randomization controls for background variables by assigning subjects so that each event is equally likely to belong to any experimental or control condition.
The placebo effect occurs when subjects are held in place in a counterbalanced design.
External validity is the presence of contamination that prevents the experimenter from concluding that a study's experimental variable is responsible for the observed effects.
When subjects are selected due to some very high or very low scores on some test and then changes on that measure are tracked in an experiment, some changes very well may occur even if there is no intervening treatment.
Pretest sensitization is created when the pretesting makes subjects less sensitive to the experimental variable.
The static group comparison uses an experimental group and a control group that are not known to be comparable with each other.
Factorial designs are experimental designs that include one independent variable, each of which are broken down into levels.
Main effects are dependent variable effects from independent variables taken together.
An interaction effect that is not in the same direction as the main effects of the variables involved (and that are diagrammatically revealed when lines drawn for each independent variable cross each other) is an ordinal interaction.
When factorial designs reveal interaction effects, main effects may not be interpreted.
Consider the above table of results from a factorial design. Assess this statement: There a main effect for variable 1 (X1).
In this example, there is a main effect for variable 2.
In this example, there is an interaction effect.
In this example, there is an ordinal interaction.
In this example it would be permissible to interpret the main effects.